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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _func-read:
+
+***********
+V4L2 read()
+***********
+
+Name
+====
+
+v4l2-read - Read from a V4L2 device
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <unistd.h>
+
+.. c:function:: ssize_t read( int fd, void *buf, size_t count )
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :c:func:`open()`.
+
+``buf``
+   Buffer to be filled
+
+``count``
+  Max number of bytes to read
+
+Description
+===========
+
+:c:func:`read()` attempts to read up to ``count`` bytes from file
+descriptor ``fd`` into the buffer starting at ``buf``. The layout of the
+data in the buffer is discussed in the respective device interface
+section, see ##. If ``count`` is zero, :c:func:`read()` returns zero
+and has no other results. If ``count`` is greater than ``SSIZE_MAX``,
+the result is unspecified. Regardless of the ``count`` value each
+:c:func:`read()` call will provide at most one frame (two fields)
+worth of data.
+
+By default :c:func:`read()` blocks until data becomes available. When
+the ``O_NONBLOCK`` flag was given to the :c:func:`open()`
+function it returns immediately with an ``EAGAIN`` error code when no data
+is available. The :c:func:`select()` or
+:c:func:`poll()` functions can always be used to suspend
+execution until data becomes available. All drivers supporting the
+:c:func:`read()` function must also support :c:func:`select()` and
+:c:func:`poll()`.
+
+Drivers can implement read functionality in different ways, using a
+single or multiple buffers and discarding the oldest or newest frames
+once the internal buffers are filled.
+
+:c:func:`read()` never returns a "snapshot" of a buffer being filled.
+Using a single buffer the driver will stop capturing when the
+application starts reading the buffer until the read is finished. Thus
+only the period of the vertical blanking interval is available for
+reading, or the capture rate must fall below the nominal frame rate of
+the video standard.
+
+The behavior of :c:func:`read()` when called during the active picture
+period or the vertical blanking separating the top and bottom field
+depends on the discarding policy. A driver discarding the oldest frames
+keeps capturing into an internal buffer, continuously overwriting the
+previously, not read frame, and returns the frame being received at the
+time of the :c:func:`read()` call as soon as it is complete.
+
+A driver discarding the newest frames stops capturing until the next
+:c:func:`read()` call. The frame being received at :c:func:`read()`
+time is discarded, returning the following frame instead. Again this
+implies a reduction of the capture rate to one half or less of the
+nominal frame rate. An example of this model is the video read mode of
+the bttv driver, initiating a DMA to user memory when :c:func:`read()`
+is called and returning when the DMA finished.
+
+In the multiple buffer model drivers maintain a ring of internal
+buffers, automatically advancing to the next free buffer. This allows
+continuous capturing when the application can empty the buffers fast
+enough. Again, the behavior when the driver runs out of free buffers
+depends on the discarding policy.
+
+Applications can get and set the number of buffers used internally by
+the driver with the :ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and
+:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctls. They are optional,
+however. The discarding policy is not reported and cannot be changed.
+For minimum requirements see :ref:`devices`.
+
+Return Value
+============
+
+On success, the number of bytes read is returned. It is not an error if
+this number is smaller than the number of bytes requested, or the amount
+of data required for one frame. This may happen for example because
+:c:func:`read()` was interrupted by a signal. On error, -1 is
+returned, and the ``errno`` variable is set appropriately. In this case
+the next read will start at the beginning of a new frame. Possible error
+codes are:
+
+EAGAIN
+    Non-blocking I/O has been selected using O_NONBLOCK and no data was
+    immediately available for reading.
+
+EBADF
+    ``fd`` is not a valid file descriptor or is not open for reading, or
+    the process already has the maximum number of files open.
+
+EBUSY
+    The driver does not support multiple read streams and the device is
+    already in use.
+
+EFAULT
+    ``buf`` references an inaccessible memory area.
+
+EINTR
+    The call was interrupted by a signal before any data was read.
+
+EIO
+    I/O error. This indicates some hardware problem or a failure to
+    communicate with a remote device (USB camera etc.).
+
+EINVAL
+    The :c:func:`read()` function is not supported by this driver, not
+    on this device, or generally not on this type of device.